1. Field of the Invention
This invention relates to unloaders for reciprocating gas compressors, and in particular to an unloader having a valve assembly with a valve guard and valve seat mounted for relative rotation. The rotatable member is driven by a constant torque motor or other power source which stalls or slips when the torque supplied is overcome by forces on the valve assembly caused by pressure differentials across the valve. Once pressures equalize, the rotatable member is free to resume rotation.
2. Description of the Related Art
In my earlier patent, U.S. Pat. No. 5,695,325, entitled Synchronized Unloader System and Method for a Gas Compressor, I disclosed an unloader system for a reciprocating gas compressor which includes an unloader valve assembly having a valve seat with multiple seat passages extending therethrough and arranged in a seat passage circle. A valve guard is rotatably mounted on the valve seat and includes a plurality of valve members arrayed in a valve circle and movable between open and closed positions with respect to the seat passages. An unloader actuation system includes a controller connected to a control system for the compressor and a stepper motor drivingly connected to the valve guard. In use, the valve guard is incrementally rotated in synchronization with the compressor crankshaft by increments corresponding to the spacing between the valve members and the seat passages. The closings of the valve members are delayed by varying amounts to achieve varying amounts of unloading.